228 CHEMICAL ENGINEERING gradation to be obtained on Wilfley and similar tables is not as sharp as is obtainable on the dry McKesson-Rice Sizer nor owing to the slight inclination is there anywhere near the capacity of the latter machine. Owing to these drawbacks riffled shaking decks have not been employed very extensively as wet grading devices but they seem In c I me d Plane Surface Morton FIG. 3.—Theoretical volumetric grading. capable of improvement and the underlying idea is suggestive. For slime and sand separation where the tonnage reaching the shaking deck is not too great, the proportion of water not too large and where both sand and slime are thoroughly in suspension the P*. r-/ FIG. 4.—Undisturbed rolling motion. FIG. 5.—Rolling motion over an obstacle. separation work on shaking riffled decks is excellent. By careful manipulation the quantity of water left in the sand can be reduced to a low amount.1 Principle of Volumetric Grading.—The essential principle governing the action in volumetric grading is the fact stated in homely language, that a large piece of rock will roll down hill faster and go farther than a small one, an experience with which every school boy is familiar. If the proper advancing motion and the 1 U. S. Patents 741,565, 1903 and 770,877, 1904 granted to August Ten Winkel show a combination of screens, riffles and deck for improving the grading work of a Wilfley table. A more complete description of tables of the Wilfley kind appears under the head of "Concentration."